1. Field of the Invention
The invention relates to a method of camouflaging defective print elements in a printer having a printhead with a plurality of print elements and capable of printing a binary pixel image, wherein each pixel of the image is assigned to a print element with which it is to be printed, and image information of a pixel that is assigned to a defective print element is shifted to nearby pixel positions where it can be printed by a non-defective print element. The invention further relates to a printer and to a computer program implementing this method. The invention is applicable, for example, to an ink jet printer, the printhead of which comprises a plurality of nozzles as print elements.
2. Discussion of the Background Art
Typically, nozzles are arranged in a line that extends in parallel with the direction (subscanning direction) in which a recording medium, e.g. paper, is transported through the printer, and the printhead scans the paper in a direction (main scanning direction) perpendicular to the subscanning direction. In a single-pass mode, commonly a complete swath of the image is printed in a single pass of the printhead, and then the paper is transported by the width of the swath so as to print the next swath or in general the single-pass mode is a mode wherein a complete line is printed by only one nozzle. When a nozzle of the printhead is defective, e.g., it has become clogged, the corresponding pixel line is missing in the printed image, so that image information is lost and the quality of the print is degraded.
A printer may also be operated in a multi-pass mode, in which only part of the image information of a swath is printed in a first pass and the missing pixels are filled-in during one or more subsequent passes of the printhead. In this case, it is in some cases possible that a defective nozzle is backed-up by a non-defective nozzle, though mostly on the cost of productivity.
U.S. Pat. No. 6,215,557 discloses a method of the type indicated above, wherein, when a nozzle is defective, the print data are altered so as to bypass the faulty nozzle. This means that a pixel that would have but cannot be printed with the defective nozzle is substituted by printing an extra pixel in one of the neighbouring lines that are printed with non-defective nozzles, so that the average optical density of the image area is conserved and the defect resulting from the nozzle failure is camouflaged and becomes almost imperceptible. This method involves an algorithm that operates on a bitmap, which represents the print data, and shifts each pixel that cannot be printed to a neighbouring pixel position. However, if this neighbouring pixel position happens to be occupied by a black pixel, anyway, pursuant to the original print data, then the extra pixel cannot be printed, and a loss of image information will nevertheless occur.
European Patent Application Publication No. 0 999 516 A2 discloses a method for generating a print mask which determines a pattern in which the pixels will be printed. This document focuses on multi-pass printing, and the main purpose of the mask is to determine which pixels are to be printed in which pass. In the mask generation process, the image information to be printed is taken into account only indirectly in the form of constraints that determine the construction of the mask. For example, such a constraint may require that a yellow pixel and a cyan pixel directly adjacent thereto are not printed in the same pass of the printhead, in order to avoid colour bleeding. This document further suggests to construct the masks in such a way that defective nozzles are backed up by non-defective nozzles.